when formatting the error message for `api_error::validation`, we
always include the caller in the error message, but in this case,
forgot to pass the `caller` to `seastar::format()`. if fmtlib
actually formats them, it would throw.
so let's pass `caller` to `seastar::format()`.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#14589